1. Field of the Invention
The present invention relates to semiconductor devices and, more particularly, to large, single-crystal semiconductor devices and methods for making such devices.
2. Description of the Prior Art
The use of a high-energy beam for growing a large single crystal from a layer of polycrystalline material on a substrate has been proposed. As the beam scans the substrate it melts the layer and, ideally, when the molten zone cools it solidifies into a single crystal.
One of the conditions required to convert the polycrystalline layer into a single crystal is the provision of a "seed"; that is, a single crystal which is in contact with the molten zone to cause it to solidify as a single crystal. There has not yet been proposed any completely satisfactory means of producing such a seed.
Various conventional energy sources, such as spot laser beam, spot electron beam, graphite strip heater and arc strip lamp, have been proposed for use in melting the polycrystalline layer to induce liquid or solid phase regrowth by epitaxial recrystallization.
However, such conventional energy sources are unsatisfactory. For example, spot beam energy sources produce a resulting recrystallized layer lacking a uniform single-crystalline structure. Conventional strip beam energy sources, such as graphite strip heaters and arc strip lamps, can damage the underlying substrate because they require a relatively long time of contact of the beam with the polycrystalline layer, which results in dissipation of an unacceptable amount of heat from the layer into the underlying substrate.
Such energy sources are also unsuitable for producing a single seed crystal. A spot laser or electron beam, impinging momentarily on a polycrystalline layer, will create a relatively small, circular molten region in the layer. However, when the region solidifies, its boundary with the rest of the layer contains small silicon crystals, which of course make the region unsuitable for use as a seed. Scanning the layer with a spot beam has also not provided a suitable seed. And the conventional strip energy sources are unsatisfactory for the same reason that they cannot be used for growing a single-crystal layer.